Dynamic sealing arrangement for turbomachines



Patented Aug. 3, 1954 DYNAMIC SEALING ARRANGEMENT FOR 'I'URBOMACHINESEarl L. Auyer, Marblehead, Mass., assigner to General Electric Company,a corporation of New York Application .ianuary 3l, 1950, Serial No.141,505

6 Claims.

This invention relates to fluid pressure energy converting devices, andmore particularly to an improved sealing arrangement in such a device.

In turbo machines of the character described, mechanical clearance mustbe provided between a stationary portion of the machine and a rotorcarrying one or more rows of blading. In order to minimize losses whicharise from the fact that a portion of the motive fluid tends to leakpast the tips of the rotor blades through a clearance space withoutdoing work on the blades, it is customary to reduce such clearance to anabsolute minimum commensurate with safe operation, It is necessary,however, to provide adequate mechanical running clearance in order toinsure against rubbing of the blades against a stationary member duringoperation which might result in destruction of the machine. In suchmachines which are intended for operation at elevated temperatures, forexample, 1000 F., or higher, the problem is further complicated becauseof certain physical properties, such as the coefficient of thermalexpansion, of the materials which must be employed in such service.

In high temperature turbine apparatus the magnitude of the clearancespace is often controlled by providing a Shroud surrounding the tips ofthe turbine blades and spaced therefrom to provide the desired amount ofoperating clearance. In operation, the temperature of the shroud willclosely approach the temperature of the motive duid iiowing past it dueto the scrubbing action of the motive fluid thereon and due to the factthat the shroud possesses relatively little mass so that the rate ofheat transfer from the fluid to the shroud is relatively high. Under thesame conditions. the turbine rotor approaches the temperature of themotive fluid in the region or" the rotor blades While in the centralportion of the rotor, the operating temperature may be as low as BOO-400F. Consequently, the average temperature at which the turbine rotoroperates is considerably less than that of the motive fluid so that, dueto the differential expansion, the magnitude of the clearance space atthe tip of the turbine blades may increase considerably over thepreselected cold value; that is, the clearance value which exists whenthe turbine rotor and the shroud are both at ambient temperature.

Accordingly, it is an object of the invention to provide a iiuidpressure energy converting apparatus having improved efficiency.

Another object is in the provision of turbine apparatus having animproved sealing arrange- 2 ment for minimizing the leakage of motivefluid past the tips of the rotor blades.

Another object is in the provision of a new and improved sealingarrangement for a turbine wherein the adverse effects of temperaturediiferentials are minimized.

Still another object is in the provision of an improved sealingarrangement wherein a jet of iiuid is employed as a sealing means and atthe same time cooling of critical parts is effected by the action of thecooling fluid.

Other objects and advantages will be apparent from the followingdescription taken in connection with the accompanying drawings, in whichFig. l is a sectional View of a turbine provided with sealing means inaccordance with the invention; Fig. 2 is an enlarged detailed View ofthe arrangement shown in Fig. l and illustrating the action of thecooling fiuid in the region of the tips of the turbine blades; Fig. 3 isan elevation of the turbine shroud; and Fig. 4 is a diagrammatic viewillustrating the flow through the turbine blades and typical pressuredistributions at the convex and concave sides of a turbine blade.

Referring now to Fig. l, motive fluid at ele vated temperature and at asuitable pressure is supplied from a suitable source (not shown) toturbine nozzles 4 which are secured to a casing 5. Nozzles 4 convert thepressure and temperature energy of the motive fluid supplied thereto tokinetic energy and direct the motive fluid against blades S at apreselected angle and a preelected velocity. Blades E5 are carried by arotor 'l which is rotatably supported by suitable bearings (not shown).The motive fluid passes through the turbine blades 6 and is thendischarged to an exhaust passage 8 formed by concentric Walls 9, IG.

In order to minimize leakage of motive fluid past the blades 6 Withoutperforming useful work thereon, an annular shroud member l l surroundstips IZ of the turbine blades and is radially spaced from the tips toprovide sufiicient echanical clearance as may be necessary to avoidrubbing of tips i2 against the shroud` Shroud Il is maintainedconcentric With the turbine rotor 'l and in such spaced relationshiprelative to the blade tips l2 by wall It. By way of example, shroud H isshown secured to Wall ld by a rivet I3, but it is to be understood thatthe shroud can be secured relative to the blade tips l2 by other typesof securing means, for ex; ample, by welding to wall it.

A feature of the invention is in the provision of a fluid dam in theclearance space i4 between the inner surface of shroud II and the bladetips I2. Such a dam is provided by introducing a jet of fluid intoclearance space I 4 in an upstream direction; that is, in a directionopposite to the ow of motive fluid through nozzles 4 and blades E. Tothis end, I provide a circumferentially extending slot I5 in shroud ii.Slot I5 is angularly disposed with respect to the inner surface ofshroud II to form a restricted fluid passageway through which a jet offluid is introduced into the clearance space I 4 at a location betweenthe leading edges Iii and the trailing edges I? of the turbine blades.It will be appreciated that by making the restricted ow passageway orslot I5 angularly disposed relative to shroud II, as shown in Figs. land 2, the fluid injected into clearance space I 4 will have asubstantial component of velocity in the upstream direction.

For reasons which will appear presently, I prefer to introduce thecooling fluid into clearance space I4 at a location upstream from thetrailing edges i'I and spaced therefrom by an amount which is of theorder of or less than 50% of the projected axial distance between theleading edges I6 and the trailing edges I'I, as indicated in Figs. l, 2and 4.

Referring now to Figs. l, 2, wall I is secured to a flange member I8,the inner surface of which is radially spaced from the outer surface ofshroud II to provide an annular chamber I9 surrounding shroud II and incommunication with the passageway formed by slot I5. Fluid underpressure is supplied to chamber I 9 from a suitable source (not shown)through a conduit portion 29. Ihe invention is well adapted for use ingas turbine powerplants of the types described in United States Patent2,432,350- Streid, and in the copending applications of Alan Howard,Serial No. 506,930, filed October 20, 1943, now Patent 2,479,573, andSerial No. 541,565, filed June 22, 1944, and assigned to the assignee ofthe present application, and in such case, sealing fluid may be providedby connecting conduit portion 26 to the compressor and thereby supplyinga portion of the compressor air ow from the powerplant to chamber I9.

In operation, motive fluid discharged by the turbine nozzles 4 isdirected against the turbine blading 6 to do useful work thereon. Afterowing through the turbine blades, the motive fluid is discharged intoexhaust passage E. Because of the mechanical clearance provided betweentips I2 and the stationary shroud I I, a portion of the motive fluidtends to leak past the tips of the turbine blades through clearancespace I4 without doing any useful work on the turbine blading. Fluidunder pressure is supplied through conduit portion 2G to chamber I9.This fluid under pressure then flows through slot or passageway I intoclearance space I 4 which is a region of lower pressure than that of thefluid supply means. Upon being discharged from the passageway formed byslots I5, the fluid forms a jet in clearance space I4 having asubstantial component of velocity in an upstream direction as clearlyindicated by the broken lines 2l in Fig. 2. The motive fluid issuingfrom nozzles 4 iiows in a downstream direction and at relatively highvelocity with respect to shroud II and blades 5, and opposite to theinward and upstream flow of fluid issuing from slots I5, thereby causingthe jet to be deflected and iiow as indicated in Fig. 2 in a downstreamdirection and ultimately to mix with the motive fluid discharged fromturbine blades 6. In opposing the inward and upstream ow of iiuid frompassageway I5, a portion of the motive fluid issuing from nozzles 4tends to flow through clearance space I4 in a downstream direction, butis deflected radially inward as shown by arrows 22, Fig. 2, and therebyis caused to do useful work on the turbine blades 6. Thus by providingan air dam, or dynamic sealing of clearance space I4, by introducing ajet of uid therein, the turbine is provided with an improved sealagainst the leakage of motive fluid past the tips of the turbine bladesand the turbine einciency is thereby improved.

In order to insure that the sealing iiuid is introduced into clearancespace I4 with a substantial component of Velocity in an upstreamdirection, and to prevent the jet of sealing fluid from penetrating toodeeply into the stream of motive fluid, I prefer to introduce thesealing l'iuid into clearance space I l at an angle a relative to theinner surface of shroud II and in an upstream direction of the order ofmagnitude of or less than degrees, as indicated in Fig. 2.

In order to consider the question of the axial location of the jet ofsealing iiuid with respect to the leading edges I6 and the trailingedges II of blades E, reference is made to Fig. 4. In addition to thefact that a portion of the motive fluid tends to flow past the tips i2through clearance space I4 in an axial direction, there is also apossibility that a portion of the motive fluid may flow in acircumferential direction past the tips I2 from the space between anadjacent pair of blades 6 through clearance space I4 and into the spacedened between a next adjacent pair of blades due to the pressuredifferential that exists across the two surfaces of the blades. Atypical pressure distribution at the convex and at the concave sides oia turbine blade is shown in Fig. 4. In addition to the loss caused byflow in a circumferential direction, such flow may oecasion furtherlosses since it may induce separation of the flow of motive fluid fromthe convex surface of the blade near the tip and near the trailing edgeIl where the static pressure is rising. In order to minimize suchlosses, I prefer to introduce the jet of uid into clearance space I4 ata location where the pressure at the concave surface of blade S isfalling rapidly so that the differential pressure for causing flow in acircumferential direction between the jet location and the trailingedges Il is minimized. Tests show that good results are obtained byspacing the jet upstream from the trailing edges I 'I by an amount ofthe order of 25% of the projected axial distance between the leadingedges I6 and the trailing edges I'I of the turbine blades. Because ofthe pressure difference i across the convex and concave surfaces nearthe leading edges, the jet location should be spaced upstream from thetrailing edge I1 by an amount not greater than of said projecteddistance. With such an arrangement, motive fiuid in the clearance spaceand upstream from the jet is deflected into the blades, and once this owis deflected into the blades it will remain there if there is little orno pressure differential between the concave and convex sides of theblades.

Another feature of the invention is in the cooling of shroud I I and inthe reduction of differential expansion tending to increase theclearance between shroud H and blade tips I2 as the turbine comes up tooperating temperature. In accordance with the invention, cooling fluidis caused to pass over the outer surfaces of the shroud, thereby coolingit, and additional cooling is obtained from action of the fluid passingthrough the passageway formed by slots l5 at high velocity. In addition,that portion of the shroud which is downstream from slots l5 iseffectively insulated from direct contact with the hot motive fluid bythe jet which forms a relatively thin iilm of cooling fluid between theinner surface of shroud l and thereby insulates it against directcontact with the hot motive fluid, as is indicated in Fig. 2. Thetransfer of heat from the motive fluid to the portions of the shrouddisposed at the upstream side of the jet of fluid issuing frompassageway l5 is eifectively reduced by the formation of a stagnantregion along the inner surface of the shroud in clearance space I4 atthe upstream side of slots l5. This region is formed due to the actionof the air dam introduced into clearance space i4 and consists of alayer of stagnant fluid interposed between the inner surface of shroudIl and the hot high-velocity motive uid issuing from nozzles 4, thuspreventing a high rate of heat transfer from the motive fluid intoshroud ll.

Thus it will be seen that my invention provides an improved sealingarrangement for turbines and thus improves turbine eiiiciency; and atthe same time it provides cooling for certain critical parts to minimizeadverse effects of differential expansion, as well as providing areduction in heat transfer from the high temperature motive uid tocritical parts of the sealing arrangement.

While a particular embodiment of the invention has been illustrated anddescribed, it will be obvious to those skilled in the art that variouschanges and modifications may be made without departing from theinvention, and it is intended to cover in the appended claims all suchchanges and modifications that come within the true spirit and scope ofthe invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates, is:

1. In a fluid pressure energy converting device including a rotorcarrying at least one row of blades, sealing means for preventingexcessive leakage of motive uid across blade tips comprising shroudmeans spaced from and surrounding said tips to form a clearance space,circumferentially extending nozzle means having an entrance portionaxially spaced downstream from the leading edges of said blades and adischarge portion axially spaced between said entrance portion and saidleading edges and carried by said shroud means for directing a jet ofiluid into said clearance space with a substantial axial component ofvelocity toward said leading edges, and means for supplying fluid underpressure to said nozzle means.

2. In an elastic fluid turbine including a rotor carrying at least onerow of axial flow blades having tip portions and upstream and downstreamedge portions, an annular shroud surrounding said tip portions andhaving an inner wall surface spaced from said tip portions and defininga clearance space therewith, said shroud having a circumferentiallyextending restricted uid now passageway therein, said passageway beingin communication with said clearance space at a location between saidleading and trailing edge portions of said blades, said passageway beinginclined at an angle less than normal to said wall surface and having adischarge portion axially spaced from an entrance portion thereof fordischarging a jet of fluid into said clearance space with a component ofvelocity toward said upstream edge portions, and means for supplyingfluid under pressure to said passageway.

3. Apparatus in accordance with claim 2 wherein said passageway isangularly disposed relative to said wali surface in the region of saidlocation at an angle not greater than 45 degrees.

4. Apparatus in accordance with claim 3 wherein said passagewaycommunicates with said clearance space at a location spaced upstreamfrom said downstream edge portions, said spacing being of the order ofmagnitude of one-fourth of the projected axial distance between saidupstream and downstream edge portions.

5. Apparatus in accordance with claim 4 wherein said spacing is notgreater than one-half of the projected axial distance between saidupstream and downstream edge portions.

6. In a turbomachine for operation at elevated temperatures including aflow passageway and a rotor carrying at least one row of blades havingtip portions and leading and trailing edge portions, an annular shroudsurrounding said tip portions and spaced therefrom to form a closeclearance space therewith, a wall portion surrounding said shroud andspaced from the outer surface thereof, said shroud having edge portionsdisposed in cooperative relation with said wall portion to denne anannular passageway surrounding said shroud, said shroud having therein acircumferentially extending second passageway between said leading andtrailing edge portions and having a first end portion communicating withthe annular passageway and having a second end portion axially spacedupstream with respect to said flow passageway from said rst end portioncommunicating with said clearance space for directing a jet of iluidinto said space with a component of velocity in an upstream direction,and means for supplying fluid to said annular passageway.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 2,390,506 Buchi Dec. 11, 1945 2,445,661 Constant July 20, 19482,457,833 Redding Jan. 4, 1949 FOREIGN PATENTS Number Country Date346,599 Germany Jan. 5, 1922 383,506 Germany Oct. 13, 1923 398,929Germany July 18, 1924

